The present invention relates to a metallocene catalyst component for use in preparing isotactic polyolefins, especially polypropylenes. The invention further relates to a catalyst system which incorporates the metallocene catalyst component and a process for preparing such isotactic polyolefins.
Olefins having 3 or more carbon atoms can be polymerised to produce a polymer with an isotactic stereochemical configuration. For example, in the polymerization of propylene to form polypropylene, the isotactic structure is typically described as having methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer. This can be described using the Fischer projection formula as follows: 
Another way of describing the structure is through the use of NMR spectroscopy. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d diad or successive methyl groups on the same side in the plane.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is described as follows: 
In NMR nomenclature, a syndiotactic pentad is described as . . . rrrr . . . in which xe2x80x9crxe2x80x9d represents a xe2x80x9cracemicxe2x80x9d diad with successive methyl groups on alternate sides of the plane.
In contrast to isotactic and syndiotactic polymers, an atactic polymer exhibits no regular order of repeating unit. Unlike syndiotactic or isotactic polymers, an atactic polymer is not crystalline and forms essentially a waxy product.
WO96/00734 relates to xe2x80x9cconstrained geometryxe2x80x9d complexes which are said to be useful as catalysts in the production of polyethylene. Some of the constrained geometry metallocenes according to WO96/00734 have a bridge between the cyclopentadienyl ring and the metal which includes a divalent heteroatom ligand. These constrained geometry metallocenes are not reported for production of polypropylenes.
In Polymer Preprints 37(2), p474 of 1996, McKnight et al report that some Group IV mono-cyclopentadienyl amido complexes may be used as catalysts in the production of polypropylene but that the polymer product is almost completely atactic.
In contrast, the present applicants have surprisingly found that isotactic polypropylene may be produced using constrained geometry metallocene catalysts, especially from a novel class of dimeric metallocene compounds.
In a first aspect, the present invention provides a metallocene compound having the general formula: CpAXMQ1Q2Cpxe2x80x2Axe2x80x2Xxe2x80x2Mxe2x80x2Qxe2x80x21Qxe2x80x22 wherein Cp and Cpxe2x80x2 are each independently a substituted or unsubstituted cyclopentadienyl moiety; M and Mxe2x80x2 are each independently a metal chosen from Group IV B transition metals and vanadium, and coordinate to Cp and Cpxe2x80x2 respectively; X and Xxe2x80x2 are each independently a substituted or unsubstituted Group VA or VIA heteroatom and coordinate to M and Mxe2x80x2 respectively; A and Axe2x80x2 are bridging groups between Cp and X and between Cpxe2x80x2 and Xxe2x80x2 respectively and are independently chosen from xe2x80x94SiRxe2x80x22xe2x80x94Oxe2x80x94SiRxe2x80x22xe2x80x94, xe2x80x94SinRxe2x80x2mxe2x80x94, xe2x80x94CnRxe2x80x2mxe2x80x94 and xe2x80x94CRxe2x80x22xe2x80x94SiRxe2x80x22xe2x80x94CRxe2x80x22xe2x80x94SiRxe2x80x22 xe2x80x94, in which each Rxe2x80x2 is independently H or hydrocarbyl having 1 to 20 carbon atoms, n is an integer in the range 1 to 4 and m=2n; each Q1, Q2 and Q1xe2x80x2 and Q2xe2x80x2 is independently a coordinating group which is hydrogen, halogen or hydrocarbyl having 1 to 20 carbon atoms and each of Q and Q1xe2x80x2 is coordinated to both M and Mxe2x80x2.
Preferably, at least one of Cp and Cpxe2x80x2 is substituted.
The metallocene compound preferably has a dimeric structure and preferably also has an active site with local C2 symmetry.
The metallocene may be used as a catalyst component for the production of a polyolefin, especially isotactic polypropylene.
In a further aspect, the present invention provides a catalyst system or use in preparing polyolefins which comprises (as a metallocene compound as defined above; and (b) an aluminum- or boron-containing cocatalyst capable of activating the metallocene compound. Suitable aluminium-containing cocatalysts comprise an alumoxane, an alkyl aluminium and/or a Lewis acid.
The alumoxanes usable in the process of the present invention are well known and preferably comprise oligomeric linear and/or cyclic alkyl alumoxanes represented by the formula: 
for oligomeric, linear alumoxanes and 
for oligomeric, cyclic alumoxane, wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C1-C8 alkyl group and preferably methyl. Generally, in the preparation of alumoxanes from, for example, aluminium trimethyl and water, a mixture of linear and cyclic compounds is obtained.
Suitable boron-containing cocatalysts may comprise a triphenylcarbenium boronate such as tetrakis-pentafluorophenyl-borato-triphenylcarbenium as described in EP-A-0427696, or those of the general formula [Lxe2x80x2xe2x88x92H]+ [B Ar1 Ar2 X3 X4]xe2x88x92 as described in EP-A-0277004 (page 6, line 30 to page 7, line 7).
The catalyst system may be employed in a solution polymerisation process, which is homogeneous, or a slurry process, which is heterogeneous. In a solution process, typical solvents include hydrocarbons with 4 to 7 carbon atoms such as heptane, toluene or cyclohexane. In a slurry process it is necessary to immobilise the catalyst system on an inert support, particularly a porous solid support such as talc, inorganic oxides and resinous support materials such as polyolefin. Preferably, the support material is an inorganic oxide in its finally divided form.
Suitable inorganic oxide materials which are desirably employed in accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides such as silica, alumina and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, or alumina are magnesia, titania, zirconia, and the like. Other suitable support materials, however, can be employed, for example, finely divided functionalized polyolefins such as finely divided polyethylene.
Preferably, the support is a silica having a surface area comprised between 200 and 900 m2/g and a pore volume comprised between 0.5 and 4 ml/g.
The amount of alumoxane and metallocenes usefully employed in the preparation of the solid support catalyst can vary over a wide range. Preferably the aluminium to transition metal mole ratio is in the range between 1:1 and 100:1, preferably in the range 5:1 and 50:1.
The order of addition of the metallocenes and alumoxane to the support material can vary. In accordance with a preferred embodiment of the present invention alumoxane dissolved in a suitable inert hydrocarbon solvent is added to the support material slurried in the same or other suitable hydrocarbon liquid and thereafter a mixture of the metallocene catalyst component is added to the slurry.
Preferred solvents include mineral oils and the various hydrocarbons which are liquid at reaction temperature and which do not react with the individual ingredients. Illustrative examples of the useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane and nonane; cycloalkanes such as cyclopentane and cyclohexane, and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.
Preferably the support material is slurried in toluene and the metallocene and alumoxane are dissolved in toluene prior to addition to the support material.
Without wishing to be bound by any theory, it is possible that the dimeric metallocene compounds described above may, in use, form corresponding monomeric compounds which act as the active catalytic species. Accordingly, the dimeric compounds may be precursors for active monomeric compounds.
In a further aspect, the present invention provides a process for preparing polyolefins, particularly isotactic polypropylenes, which comprises contacting the catalyst system with at least one olefin in a reaction zone under polymerization conditions. The olefin is preferably propylene.
In a further aspect, the present invention provides use of a catalyst comprising a metallocene compound for the production of isotactic polypropylene, wherein the metallocene compound has the general formula CpAXMQ2, in which Cp is a cyclopentadienyl moiety substituted so that when in use isotactic polypropylene is produced; M is a metal chosen from Group IVB transition metals and vanadium, and coordinates to Cp; X is a substituted or unsubstituted Group VA or VIA heteroatom which coordinates to M; A is a bridging group between Cp and X which is chosen from xe2x80x94SiRxe2x80x22xe2x80x94Oxe2x80x94SiRxe2x80x22xe2x80x94, xe2x80x94SinRxe2x80x2mxe2x80x94, xe2x80x94CnRxe2x80x2mxe2x80x94 xe2x80x94CRxe2x80x22xe2x80x94SiRxe2x80x22xe2x80x94CRxe2x80x22xe2x80x94SiRxe2x80x22xe2x80x94, in which each Rxe2x80x2 is independently H or hydrocarbyl having 1 to 20 carbon atoms, n is an integer in the range 1 to 4 and m=2n; and each Q is independently hydrogen, halogen or hydrocarbyl having 1 to 20 carbon atoms. The metallocene compound preferably has an active site with local Cl symmetry when the active catalyst is in the form of a monomer.
Preferably, each cyclopentadienyl moiety is a substituted or unsubstituted indenyl, more preferably a substituted or unsubstituted benzoindenyl. A preferred substitution position is position 2 in the cyclopentadienyl ring, the substituent of which may be hydrocarbyl having 1 to 20 carbon atoms, such as methyl.
M is preferably Zr.
Each heteratom is preferably nitrogen, phosphorus, oxygen or sulphur so as to provide a suitable divalent ligand for constraining the geometry of the metallocene. The heteroatom is typically substituted with H, hydrocarbyl having 1 to 20 carbon atoms or silyl and is preferably nitrogen. Each A and Axe2x80x2 is preferably SiRxe2x80x22 wherein each Rxe2x80x2 is preferably methyl.
In a further aspect of the invention, there is provided use of a metallocene compound as described above wherein the metallocene compound is formed from a dimeric metallocene compound also as described above.